1. Field of the Invention
The present invention pertains to the field of blowout preventer valves of the type used for sealing oil wells, and is more particularly directed to a wireline blowout preventer useful for sealing a well with a braided wireline extending through the preventer into the well.
2. State of the Prior Art
In general, existing blowout preventers have a valve body through which extends a vertical well bore with mounting flanges at each end for attachment to other well head apparatus. The valve body also includes a pair of aligned horizontal ram guides which intersect and open into the well bore at right angles thereto. Within each ram guide is a ram movable between an open, spaced apart position in which the rams are retracted into the ram guides and out of the well bore, and a closed position in which the rams are brought together into opposing engagement within the well bore to create a seal which closes the well bore. The movement of the rams between the open and closed positions may be accomplished by manual and/or hydraulic means.
Blowout preventers of the ram type have been in use since the 1920's. Early devices were blind ram blowout preventers used for sealing off wells without any piping or lines extending into the well. Later, blowout preventers were developed for sealing off well pressure with a pipe captive and extending through the blowout preventer into the well.
Wireline tools and operations came into increasing use during the middle of this century, and a need developed for wireline blowout preventers, i.e., blowout preventers capable of sealing the well with a wireline extending into the well. As working pressures at well heads became higher, and the use of braided wire became more common, the problem of creating an effective seal between the rams and the irregular surface of the braided wireline was addressed. Wireline operators found that by stacking two blowout preventers together, the bottom one upside down, and the top one right side up, an intermediate cavity was created between the top and bottom ram pairs into which a heavy grease could be injected. The grease would fill the voids and spaces between individual wires in the braided wire surface to provide a better seal between the braided wire and the blowout preventer rams. While this made it possible to seal higher well pressures, cleaning such a double blowout preventer following a sealing operation is a difficult and time-consuming task, due to the quantity of grease required to fill the relatively large cavity in the device and its tendency to gum-up the blowout preventer mechanism. Proper and safe maintenance of such devices remains a problem.
The dual ram-pair blowout preventers used in the past to seal high-pressure wells are too difficult to maintain and clean, and it is therefore desirable to provide a reliable single blowout preventer, i.e., one comprising a single pair of rams. Such single ram-pair wireline blowout preventers are known and commercially available from a number of manufactures of oil tooling.
Currently available single ram pair wireline blowout preventers typically depend on elastomeric seal inserts in the ram faces which are deformed by pressure against the braided wireline captive between the ram faces, such that the elastomer material tends to conform to and fill the crevices in the wireline surface. While such simple blowout preventers are adequate for relatively low well pressures, the seal formed between the elastomer and the wireline surface is not capable of withstanding higher pressures and heavier duty applications still require stacking of multiple single ram-pair units, with or without grease injected in-between to effectively seal off a well during wireline operations.
A somewhat improved single ram pair wireline blowout preventer is disclosed by Williams, Jr. in U.S. Pat. No. 4,227,543. In the Williams' device a plastic sealant material is injected through ports in the valve body opening into a cavity defined between the ram faces by grooves which extend across each of the ram faces in a direction perpendicular to both the ram guides and the well bore of the blowout preventer. The plastic sealant supplements the deformable seal inserts in the ram faces to make an improved seal against a pipe extending through the blowout preventer and captive between the rams. The Williams device requires that grease injection ports be provided in the valve housing in alignment with the grease conduit formed between the ram faces in the closed position, unnecessarily complicating the manufacture of the device. Williams requires a greater amount of "grease" packing material than the present invention, and also injects the grease into a channel extending diametrically across the well bore leading to more widespread contamination of the valve mechanism with grease than is necessary, thus increasing the difficulty and cost of maintenance of the same.
A continuing need therefore exists for a simple and effective wireline blowout preventer which is dependable at high well pressures, and yet which is easy to clean following a sealing operation and economical to maintain.